In a hard disk drive system, also known as a rigid or Winchester disk drive system, it is advantageous to use rotary or linear voice coil motors as the driving element of a head actuator which moves the magnetic recording/reproducing heads across the disk surfaces. A voice coil motor is responsive to a current supplied therethrough to drive the head actuator with an acceleration proportional to the magnitude and direction of the current. The disk drive system uses the voice coil motor to perform, inter alia, a track access operation in which the magnetic heads are moved rapidly across the disk surfaces, skipping over, for example, hundreds of tracks, to quickly reach a desired track at which recording or reproduction will begin. In order to complete the track access operation as quickly as possible, the head actuator should be driven with maximum acceleration, and rotary voice coil motors have the desirable ability to provide high acceleration with small resistance, for example arising from back emf voltage.
As a result of this high acceleration, the maximum self-limiting velocity of the head actuator can be very high, and this creates a problem. In hard disk drive systems, the magnetic heads "fly" over the surfaces of the disks on an air cushion generated by the rotation of the disks themselves. When power is turned off, the head actuator is driven to move the magnetic heads to an information-free parking or landing zone on which they may rest without destroying information, which is recorded only in other areas of the disk. The magnetic heads should come to rest relatively softly, i.e. slowly, to the surface of the disk, or they may be damaged on impact. Typically, the head actuator brings the heads quickly to the parking zone in case of error or loss of power, and generally a crash stop pin is provided to limit further movement of the head actuator once it reaches the parking zone. It has been found, however, that if the head actuator is moving too fast when it hits the crash stop pin in a so-called over-velocity error condition, the negative acceleration of the actuator is excessive and causes the magnetic heads to crash to the disk surface.